Pre-drive circuit

ABSTRACT

A pre-drive circuit performs on/off control of a switching transistor element (Q 1 ) through a pulse transformer (T) having two primary windings (P 1 , P 2 ). Switching elements (Q 2 , Q 3 ) are respectively connected to the primary windings. Feedback circuits (D 1 , D 2 ) connect signal output terminals of the respective switching elements (Q 2 , Q 3 ) to control terminals of the opposite switching elements (Q 3 , Q 2 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a pre-drive circuit for use in a switchingregulator, a DC/DC converter, etc.

2. Description of the Related Art

A pre-drive circuit which performs the on-off control of a switchingtransistor element through a pulse transformer is one which on/offcontrols the power transistor of a switching circuit by the use of twodrive transistors, as illustrated in FIG. 4.

With reference to FIG. 4, a switching circuit 1 on/off controls a powersource PS for a load L by means of a power transistor Q₁ and isconnected to a control circuit 3 through a pre-drive circuit 2.Connected across the base and emitter of the power transistor Q₁,through a parallel circuit consisting of a resistor R₁ and a capacitorC, is a secondary coil S of a pulse transformer T. On the primary sideof the pulse transformer T, a transistor Q₂ is connected to a firstprimary coil P₁, and a transistor Q₃ is connected to a second primarycoil P₂.

In turning on the power transistor Q₁, a signal output transistor Q₅ ofthe control circuit 3 is turned off to turn on the transistor Q₂. Whenthe transistor Q₂ is turned on, voltages in polarities indicated bysolid lines in the figure are generated in the pulse transformer T, andcurrent flows to the base of the power transistor Q₁ to bring this powertransistor Q₁ to the on state.

In turning off the power transistor Q₁, the signal output transistor Q₅is turned on which turns off the transistor Q₂, while at the same time,a signal output transistor Q₄ is turned off to turn on the transistorQ₃. The base charge of the power transistor Q₁ is extracted by voltagesin polarities indicated by the broken lines in the FIGURE, therebyturning on this power transistor Q₁.

With such a prior art pre-drive circuit, the transistor Q₂ is turned offand the transistor Q₃ is simultaneously turned on in order to turn offthe power transistor Q₁. In this regard, the conventional transistorsinvolve a delay in terms of the response of collector current to basecurrent, and when control signals SA and SB to be applied to thepre-drive circuit 2 are provided with exactly opposite phases, thetransistors Q₂ and Q₃ are turned on simultaneously. At this moment,excess currents flow through both the transistors Q₂ and Q₃, resultingin the useless generation of heat. This makes it necessary to delayslightly the time at which the transistor Q₃ turns on.

Moreover, the prior art involves problems in terms of the currentefficiency and circuit arrangement thereof. That is, the control circuit3 requires the two signal output transistors Q₄ and Q₅ in order tosupply the respective transistors Q₂ and Q₃ with the control signals SAand SB.

SUMMARY OF THE INVENTION

The present invention has been divised in order to solve the foregoingproblems, and its object is to provide a pre-drive circuit in whichfeedback circuits are cross-connected to transistors serving asswitching elements, wherein after one of the transistors is turned off,the other transistor is turned on, thereby simplifying the arrangementof a control circuit and preventing the useless generation of heat.

According to the present invention, it is possible to provide apre-drive circuit wherein the on/off control of a switching transistorelement is performed through a pulse transformer having two primarywindings to which switching elements are respectively connected, thepre-drive circuit comprising feedback circuits which cross-connect thesignal output terminals of the respective switching elements and thecontrol terminals of the opposite switching transistors.

With the pre-drive circuit of the present invention, a corresponding oneof the feedback circuits is connected from the signal output terminal ofone switching element to the control element of the other switchingelement, whereby after one switching element has completely turned off,the other switching element turns on. Accordingly, excess currents canbe prevented from developing in the pre-drive circuit and thearrangement of the control circuit can be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a pre-drive circuit and its peripheralcircuits showing an embodiment of the present invention;

FIG. 2 is a chart showing the operation timing of the circuit of theembodiment illustrated in FIG. 1;

FIG. 3 is a circuit diagram showing another embodiment of the presentinvention; and

FIG. 4 is a circuit diagram showing an example of a prior-art circuitarrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described in detailwith reference to the drawings.

FIG. 1 shows a pre-drive circuit illustrative of a first embodiment ofthe present invention. Although shown in a simplified form in thefigure, a switching circuit 1 to be driven by a pre-drive circuit 2includes a power transistor Q₁, etc. This power transistor Q₁ is turnedon and off by current which is supplied from a pulse transformer T.

The emitter of the power transistor Q₁ is connected to the minus side ofthe power source PS of the switching circuit 1 and is also connected toone end of the secondary coil S of the pulse transformer T. The otherend of the secondary coil S is connected to the base of the transistorQ₁ through a parallel circuit consisting of a resistor R₁ and acapacitor C₁.

On the primary side of the pulse transformer T, two primary coils P₁ andP₂ are arranged in opposite phases to each other. The collector of aconventional bipolar transistor Q₂ is connected to the first primarycoil P₁, while the drain of a MOS-type FET Q₃, which functions as thesecond switching element, is connected to the second primary coil P₂. Inaddition, a diode D₁, which functions as a feedback circuit, isconnected in the reverse direction from the collector terminal of thetransistor Q₂ to the gate terminal of the FET Q₃, while a diode D₂,which functions as a feedback circuit, is connected in the reversedirection from the drain terminal of the FET Q₃ to the base terminal ofthe transistor Q₂. Further, a diode D₃ is connected in the forwarddirection between the emitter of the transistor Q₂ and ground, wherebythe potential difference of the transistor Q₂ relative to ground israised by an amount corresponding to the forward potential differenceacross the diode D₃. Meanwhile, a control circuit 3, which deliverscontrol signals to the pre-drive circuit 2, has a signal outputtransistor Q₄, the emitter and collector of which are respectivelyconnected to the base of the transistor Q₂ and the gate of the FET Q₃.Thus, the control circuit applies a predetermined voltage from a voltagesource V₁ through a resistor R₃ to a node between the collector of thetransistor Q₄ and the gate of the FET Q₃, to determine the base currentof the transistor Q₂ when the transistor Q₄ is on, and supplies asufficient voltage to the gate of the FET Q₃ when the transistor Q₄ isoff, to turn on this FET Q₃.

FIG. 2 exemplifies on and off operation timing in the circuit of thefirst embodiment. FIG. 2(a) shows the relative state of the signaloutput transistor Q₄ of the control circuit 3, (b) the relative state ofthe FET Q₃, (c) the state of the relative transistor Q₂, and (d) therelative state of the power transistor Q₁.

When the transistor Q₄ is turned on with the intention of turning on thepower transistor Q₁, currents attempt to flow from the resistor R₃ andthe gate of the FET Q₃ through the transistor Q₄ to the base of thetransistor Q₂. However, since current flows to the drain of the FET Q₃through the diode D₂ while the FET Q₃ is on, the transistor Q₂ does notturn on. When the FET Q₃ turns off owing to the extinction of the gatecharges thereof, the current through the diode D₂ fails to flow, and thecurrent flows to the base of the transistor Q₂ instead, so that thistransistor Q₂ turns on. Thus, voltages in polarities indicated by thesolid lines in FIG. 1 are generated in the pulse transformer T, and thepower transistor Q₁ is turned on.

On the other hand, when the transistor Q₄ is turned off with theintention of turning off the power transistor Q₁, the transistor Q₂ hasits base current cut off and turns off after elapse of a period of timerequired for the extinction of the charges of the base thereof. Duringthe time that the transistor Q₂ is on, current from the resistor R₃flows to the collector of the transistor Q₂ through the diode D₁, andthe gate voltage of the FET Q₃ remains low in spite of the off state ofthe transistor Q₄ ; hence, the FET Q₃ does not turn on. When thetransistor Q₂ turns off to raise the collector voltage thereof, the FETQ₃ has its gate voltage increase and turns on. Thus, voltages inpolarities indicated by the broken lines in the figure are generated inthe pulse transformer T, and the power transistor Q₁ is turned off.

FIG. 3 is a circuit diagram showing another embodiment of the presentinvention. In this embodiment, a power transistor Q₁ ' of a switchingcircuit 11 to be controlled by a pre-drive circuit 21 is a MOSFET, andthe circuit arrangement of a FET Q₃ connected to the secondary primarycoil P₂ differs from that in the case of FIG. 1.

Operation at the time the power transistor Q₁ ' attempts to turn on isthe same as in the case of FIG. 1. Further, regarding operation at thetime the power transistor Q₁ ' attempts to turn off, operation until theFET Q₃ turns on is the same as in the case of FIG. 2. When the FET Q₃turns on, both ends of the secondary coil S are equivalentlyshort-circuited, so that the voltage at the gate of the power transistorQ₁ ' connected to the secondary coil S becomes about zero, turning offthe power transistor Q₁ '.

In the pre-drive circuit illustrated in each of the embodiments, thegate voltage required to turn on the FET Q₃ is usually 2 V or above.Hence, the emitter and collector of the single signal output transistorQ₄ are respectively connected to the transistor Q₂ and the FET Q₃,whereby the transistor Q₂ and the FET Q₃ can be prevented from beingturned on simultaneously by the transistor Q₄. This prevents the uselessgeneration of heat in the pulse transformer T and the transistor Q₂ aswell as the FET Q₃.

Though two embodiments of the present invention have been described, theinvention is not limited thereto and can be modified in a variety ofways without departing from the scope of the claims.

When used in a switching regulator, a DC/DC converter or the like, thepre-drive circuit of the present invention can improve the currentefficiency and circuit arrangement thereof.

I claim:
 1. A pre-drive circuit, for on/off control of a switchingtransistor element, including a pulse transformer having two primarywindings, comprising:first and second switching elements, each having asignal output terminal and a control terminal, and one being associatedwith each of the primary windings, which control currents flowingthrough the respective primary windings; and feedback circuits whichcross-connect the signal output terminals and the control terminals ofsaid first and second switching elements.
 2. A pre-drive circuitaccording to claim 1, wherein each of said feedback circuits isconstructed of a diode which is connected in a forward direction withrespect to a corresponding control signal, and in that after one of thesaid first and second switching elements has turned off, the other isturned on.
 3. A pre-drive circuit according to claim 1, wherein abipolar transistor is employed as one of said first and second switchingelements, and an insulated-gate field-effect transistor is employed asthe other.
 4. A pre-drive circuit for controlling switching of atransistor element, comprising:means, having first and second primarywindings and a secondary winding, for performing on/off switching of thetransistor element; first and second switching means, each having acontrol terminal and a signal output terminal, for controlling thecurrents flowing through the first and second primary windings,respectively; and feedback means for cross-connecting the controlterminal of each of said first and second switching means to the signaloutput terminal of said first and second switching means.
 5. A pre-drivecircuit according to claim 4, wherein said feedback means comprise firstand second circuits, each having a diode, such that after one of saidfirst and second switching means is turned off, the other is turned on.6. A pre-drive circuit according to claim 4, wherein said firstswitching means comprises a bipolar transistor and said second switchingmeans comprises an insulated-gate field effect transistor.
 7. Apre-drive circuit according to claim 5, wherein said first and secondprimary windings having opposite phases, so that when said first primarywinding is turned on by turning on said first switching means, thetransistor element is turned on, and when said second primary winding isturned on by turning on said second switching means, the transistorelement is turned off.